As a conventional technique, there has been proposed a rotation detection device for detecting the rotation angle of a rotating body by counting the passage of gear teeth provided on the rotating body (for example, see JP 2015-215342 A).
The rotation detection device disclosed in JP 2015-215342 A includes a cylindrical bias magnet, three magnetoresistive elements to detect the magnetic field which changes when the gear teeth provided on the rotating body passes as the rotating body rotates, a detecting section for detecting differential signals each from two outputs of the three magnetoresistive elements, an amplitude adjusting section for adjusting the amplitudes of the differential signals detected by the detecting section so as to cause the amplitudes of the respective differential signals to coincide with each other, and a difference value acquiring section for acquiring a difference value of each of the differential signals whose amplitudes have been adjusted by the amplitude adjusting section, a determination circuit section for generating a binary signal based on a threshold value from the difference value acquired by the difference value acquiring section, and counts the passage of the gear teeth on the basis of the waveform of the binary signal generated by the determination circuit section and detects the rotation angle of the rotating body. When the three magnetoresistive elements are arranged to be tilted with respect to the symmetry of the magnetic flux generated by the cylindrical bias magnet, the amplitude of one of the differential signals is large and that of the other is small, so that the amplitudes are different. However, since the amplitudes are adjusted to coincide with each other by the amplitude adjusting section, the binary signal accords with the passage of the gear teeth.